Westminster asserts that under the Scotland Act, the UK government would need to make a so-called section 30 order to permit a referendum. Such an Order in Council was issued in the wake of the Edinburgh agreement between Westminster and Holyrood in advance of the 2014 poll.
The government under Prime Minister Boris Johnson asserts that, in the interest of stability, important decisions of this kind should only be taken once in a generation. The Scottish First Minister, Nicola Sturgeon, counters that a section 30 order cannot be reasonably refused. Indeed, her governing Scottish National Party (SNP) has pledged to hold an independence referendum still this year, after the May elections in Scotland.
The assertion that referenda can only happen once in a generation is not backed up by practice elsewhere. For instance, only last year New Caledonia repeated its referendum on independence from France merely two years after the initial poll of 2018. Continued association with Paris won on both occasions.